1. Field of the Invention
The disclosures discussed herein relate to an image forming apparatus.
2. Description of the Related Art
In the commercial printing industry, a printing technology called “Print On Demand” (POD) has become popular for printing small lots and various variable data along with progress in shifting from a offset printing press to an image forming apparatus employing an electrophotographic system and the like. Since such an image forming apparatus employing an electrophotographic system meets a wide variety of needs as described above, accuracy in alignment of two surfaces, uniformity of images comparable to those provided by the offset printing press, and the like may increasingly be required.
Factors of misalignment of two surfaces may be roughly divided into a) registration errors in a vertical direction and a horizontal directions; b) a skew error between a recording medium and a printing image; and c) image length expansion and contraction in transferring a toner image. Further, in an image forming apparatus having a fixing device, misalignment of two surfaces may occur due to an image magnification error caused by expansion and contraction of a recording medium resulting from heating.
For example, Japanese Laid-open Patent Publication No. 2004-347842 discloses a technology for preventing misalignment of images printed on two surfaces of a sheet by detecting dimensions in a sheet transporting direction and in a sheet width direction orthogonal to the sheet transporting direction before and after printing the image on the sheet serving as a recording medium, and correcting the images based on changes in the dimensions computed based on detected results.
However, in a method of correcting image magnification based on the measured results of the sheet size before and after printing the image on one side of the sheet, such as that disclosed in Patent Document 1, it is typical to measure a dimension in the width direction of the sheet at a predetermined timing based on a counter value, or the like after the detection of a front end of the transported sheet.
In such a method, in an image forming apparatus employing a switchback system to turn the sheet the other way up or back to front after printing an image on one surface of the sheet, a dimension measuring position in a width direction before printing the image on one surface of the sheet may differ from a dimension measuring position in the width direction after printing the image on that surface of the sheet. In such a case, even though the magnification of an image to be printed is corrected based on width dimensions measured at different positions in the sheet transporting direction before and after the printing the image on one side of the sheet, the misalignment of two surfaces of the sheet is not necessarily corrected due to the ratio of expansion and contraction varying at different positions in the sheet transporting direction.